1. Field of the Invention
The present invention pertains generally to the field of manufacturing and tuning radio frequency (RF) power transistors and amplifiers.
2. Background
The use of RF power transistor devices as signal amplifiers, e.g., in wireless communication applications, is well known. With the considerable recent growth in the demand for wireless services, such as personal communication services, the operating frequency of wireless networks has increased dramatically and is now well into the gigahertz frequencies. For such high frequency amplifier applications, laterally diffused metal oxide semiconductor (LDMOS) transistors are usually preferred, but Bi-Polar and other transistor types may also be used.
In a typical deployment, a RF power transistor comprises a plurality of electrodes formed on a semiconductor (silicon) die, with each electrode comprising a plurality of interdigitated transistors. With a LDMOS device, the individual transistors of each electrode are connected to respective input (gate) and output (drain) terminals located on the top surface of the die, with the underside of the die comprising a common element (source) terminal. The die is attachedxe2x80x94e.g., by a known eutectic die attach processxe2x80x94to a substrate serving as both a heat sink and common element ground reference. Further input and output transmission elements, such as one or more matching capacitors and/or shunt inductors may also be attached to the substrate surface. To complete the amplification circuit, respective input and output RF signal leads are electrically coupled by a plurality of bond wires to the respective input and output transmission elements, which in turn are electrically coupled to the respective input and output electrode terminals on the transistor die.
Production of RF power transistor amplifiers on a large scale basis is traditionally a problem, because of natural variables that the individual elements possess. In particular, the transistor devices have natural variances in input capacitance, gain and signal phase shift. Thus, in commercial implementations, significant time and effort is needed to first characterize a particular transistor device over the range of expected operating voltages, and then attempt to build many devices just like it which deliver a desired output gain and phase. However, due to the transistors"" and various other elements, gain and phase variations over identical operating voltages, the ability to successfully tune transistor devices is limited.
Consistent performance of high frequency RF power transistors is, thus, problematic due to their natural variations. These variances must be compensated for in the amplification circuits to achieve reliable and consistent performance. For example, DC biasing and temperature compensation circuits are traditionally employed at the input of the device to compensate for inherent differences between individual power transistor devices and for changes in temperature during operation.
In addition to external matching of the input and output circuits, internal matching of the input and output transistor electrode terminals on the die to the input and output leads is also highly desirable for proper operation of a power transistor and amplifier devices. Unlike external device matching, however, internal matching to the respective input and output electrode terminals on the die is done at relatively low impedance levelsxe2x80x94e.g., one tenth of an ohm to three ohms at the input terminal and five to eight ohms at the output terminal. As will be appreciated by those skilled in the art, the actual impedance at the respective electrode input and output terminals is a function of operating power and frequency, as well as the number of electrode cells and dies of the particular device.
Also known in the art are various forms of non-recoverable tuning techniques, e.g., laser trimming of capacitors in a matching or blocking circuit. However, non-recoverable tuning can be undesirable because it is performed before it is known whether a particular die functions as designed and generally at a point in the process where the amplifier device or circuit is nearly complete. In addition, laser trimming is irreversible, which makes rework of the particular device or circuit difficult or impossible.
In accordance with a first aspect of the invention, preferred methods of manufacturing and tuning power transistor devices and circuits are provided.
In one embodiment, a method for manufacturing a power transistor circuit includes securing a die to a substrate, the die comprising a transistor having an input terminal and an output terminal. One or more performance characteristics of the transistor are measured. Using one or more wire sets, the transistor input terminal is electrically connected to one or more input matching elements and an input signal lead. The impedance of the one or more wire sets, (as determined by selecting a desired number and/or length of the wires in each set, is selected based at least in part on the measured transistor performance characteristic(s). Similarly, using one or more additional wire sets, the transistor output terminal is electrically connected to one or more output matching elements and an output signal lead, wherein the impedance of the additional wire sets is selected based at least in part on the measured transistor performance characteristic(s).
Measurement of the transistor performance characteristic(s) may be performed by electrically coupling the transistor(s) to a known test network prior to attachment of the input/output transmission path connection wires. The performance characteristic(s) may include, by way of non-limiting examples, inherent input or output capacitance, impedance, gain flatness and signal phase shift. Alternately, the performance characteristic(s) may be measured by assembling the power transistor circuit or amplifier device, excepting one or more wires in the input and/or output transmission paths, measuring performance of the nearly completed device or circuit at the input and output leads, and then tuning the performance by selecting the number and/or length of additional input and/or output transmission path wires used to complete the device or circuit.
It is further contemplated by the invention to provide power transistor devices and circuits manufactured by the methods taught herein.
In one embodiment, a power transistor circuit includes a die secured to the substrate, the die comprising a transistor having an input terminal and an output terminal. An input lead, one or more input matching elements, output lead, and one or more output matching elements are also secured o the substrate. A plurality of wire sets electrically couple the transistor input terminal to the one or more of the input matching elements and input signal lead. Similarly, one or more additional wire sets electrically couple the transistor output terminal to the one or more output matching elements and the output lead. The impedance of at least one of the respective wire sets, as determined by selecting a desired number and/or length of the wires in the set, is selected based at least in part on a performance characteristic of the transistor measured after the die was secured to the substrate.
Other aspects and features of the invention will become apparent from consideration of the following detailed description taken in conjunction with the accompanying drawings.